Is \x a primitive anywhere?

Question

It is claimed by Marc van Dongen at Use TikZ foreach variable in node that \x is a "TiKZ primitive". Does TikZ have primitives? I can't understand why on earth any package would make \x a primitive, and I assume that outside TeX engines/platforms only LuaTeX can make a package/user command equivalent to a primitive. Please can anyone tell me the meaning of the \x primitive? I can't see it defined outside or within tikzpicture!

EDIT

I have now seen what is going on. Although TikZ has no primitives, Marc van Dongen has a point. From cjorssen's example,

\draw plot[samples=10,domain=-5:5] (\y,{(\y)^2})

fails, while

\draw plot[variable=\y,samples=10,domain=-5:5] (\y,{(\y)^2})

works. In my humble opinion, PGF shouldn't have used a generic macro like \x as a default scratch macro, even in a local group or with a restored stack. It may be convenient for users, but, as we've seen, it is not the best choice of a scratch variable.

Answer 1

\x is not a tikz "primitive" as far as I understand. It can be used in the plot operation but it is behind the scenes used by \foreach. See in the following example: the two \draw commands do more or less the same (see lines 388- in pgfmoduleplot.code.tex and lines 2899- in tikz.code.tex).

\documentclass{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
  \draw plot[samples=10,domain=-5:5] (\x,{(\x)^2});
  \draw[red] (-5,{(-5)^2}) \foreach \x in {-4,...,5} {-- (\x,{(\x)^2})};
\end{tikzpicture}
\end{document}

The \foreach variable (here \x) is group protected so the fact that \x would be defined elsewhere is not a problem here.

I'd say that even if the implementation of the plot operation change in the future, tikz/pgf developers would take care of protecting the definition of \x.